R help - Apr 2006 - Polygon-like interactive selection of plotted points

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Hi all,

I have the following problem/question: Imagine you have a two- 
dimensional plot, and you want to select a number of points, around  
which you could draw a polygon. The points of the polygon are defined  
by clicking in the graphics window (locator()/identify()), all points  
inside the polygon are returned as an object.

Is something like this already implemented?

Thanks a lot in advance,

Florian

On Wed, 2006-04-26 at 18:13 +0100, Florian Nigsch wrote:
> [Please CC me for all replies, since I am not currently subscribed to  
> the list.]
> 
> Hi all,
> 
> I have the following problem/question: Imagine you have a two- 
> dimensional plot, and you want to select a number of points, around  
> which you could draw a polygon. The points of the polygon are defined  
> by clicking in the graphics window (locator()/identify()), all points  
> inside the polygon are returned as an object.
> 
> Is something like this already implemented?
> 
> Thanks a lot in advance,
> 
> Florian
I don't know if anyone has created a single function do to this (though
it is always possible). 

However, using:

  RSiteSearch("points inside polygon")

brings up several function hits that, if put together with the above
interactive functions, could be used to do what you wish. That is, input
the matrix of x,y coords of the interactively selected polygon and the
x,y coords of the underlying points set to return the points inside or
outside the polygon boundaries.

Just as an FYI, you might also want to look at ?chull, which is in the
base R distribution and returns the set of points on the convex hull of
the underlying point set. This is to some extent, the inverse of what
you wish to do.

HTH,

Marc Schwartz